In general, this invention relates to a photographic camera having an exposure control system of the type that operates in cooperation with an adjustable-diaphragm lens used with the camera to controllably change diaphragm value so as to provide proper exposure. More particularly, it relates to an improved system arrangement involving a controllable brake mechanism.
Various types of automatic exposure control systems are well known in the photographic camera art. One such well known type of system referred to as the diaphragm value control type. In this type of system, the diaphragm value of an objective lens used with the camera is automatically controlled in accordance with exposure factors such as brightness of an object to be photographed, film sensitivity, and preset shutter speed.
During an exposure control operation performed by such a system, the amount of light coming through the lens aperture from the object to be photographed is determined with the objective being progressively stopped down from the opened state thereof and, when the appropriate diaphragm value is reached, the operation of stopping down is suspended so that proper exposure results when the shutter operates. A significant drawback of a conventional arrangement of such system will now be described. When an objective provided with a diaphragm mechanism of well known art is employed in the photographic camera of this type, it has been impossible to control the diaphragm mechanism with a high precision. One reason for this is that the time available to complete the exposure control operation, i.e., the time elapsing from the start to completion of stopping down, is excessively short. As a possible approach to this problem, one might try simply to associate a drag mechanism with the actuating members which are, in turn, operatively associated with operation of stopping down.
However, if such a drag mechanism is provided and so designed as to keep the speed of stopping down sufficiently slow to achieve the desired control with an adequate precison, the time of operation would be excessively long in many cases. This is particularly so in the cases in which it is desired to stop the lens down to the minimum diaphragm value or adjacent thereto. In other words, the time lag between depression of the shutter release button and actual opening of the shutter will be to long for practical picture-taking purposes. If the drag mechanism were so designed as to make its effect weaker with intent to overcome this problem, the particular advantage of providing such drag mechanism would be lost, although the time of operation could certainly be shortened.
In view of the foregoing, provision of a brake mechanism has been proposed so that diaphragm control might be achieved with a high precision in spite of relatively short total time of operation, but such a control has required a complicated circuit arrangement and has been encountered by various problems such as those with respect to cost and adjustment.